1. Related Field
The present invention relates generally to a device to control the motion of a long product, such as a steel bar or rod, moving with high linear speed, in a manufacturing process, such as rolling.
2. Background of the Invention
Certain manufacturing processes, such as rolling, drawing and extrusion are utilized to reduce the cross sectional dimensions of metal products through mechanical contact between the metal workpiece and different tools such as rolls and dies. These manufacturing processes are continuous, or substantially continuous, processes and are herein collectively referred to as “reducing processes.” This invention applies to metal products that are commonly referred to as long products or bars and/or rods. These metal products move along a longitudinal axis in a manufacturing process and will be referred to hereinafter as a “bar” or “bars.”
A bar is different than a metal slab, bloom or strip, all of which are known as flat products. The cross section of a bar has a smaller circumference/cross-section-area ratio than flat products and the bar may rotate/twist about its longitudinal axis while moving forward longitudinally. The bar shapes shown in FIG. 2, for example, have a ratio of circumference to cross-section equal to or smaller than 4.25 when the cross sectional area is unity for the given shape. The shapes of the cross-section of a metal bar shown in FIG. 2 include round, oval, or polygonal.
In the hot rolled steel industry, the length to circumference ratio of the bar after it is reduced is typically over 10 and the length to cross-section critical dimension, such as the diameter of a round bar, is over 30. Furthermore, the bar frequently travels through the reducing process at high speed and high temperature.
The manufacturing process is designed to move the bar along a predetermined, ideal path line (herein referred to as the “bar path”) through various reducing mechanisms that apply the appropriate mechanical reducing forces to the bar in a controlled, consistent manner. It is desirable to constrain the bar to the bar path by controlling the bar's non-axial motion (herein referred to as “non-axial motion”) as it moves along the bar path through the reducing mechanisms.
Guides:
A single hot steel rolling line normally produces bars with a range of different diameters. For example, a single hot rolling bar mill could produce bars with diameters ranging from 5 mm to 25 mm. The cost of changing the line to produce a bar with a different diameter from the one currently being rolled is partly a function of the number of different pieces of equipment that have to be changed in order to produce the new diameter.
Steel mills use devices (herein referred to as “guides”) to control the bar's motion. The guides have a guidance path (herein referred to as the “guidance path”) that acts to constrain the motion of the bar and force it onto the bar path. The diameter of the guidance path cannot be either smaller, or much larger, than the diameter of the bar or the guide will not function properly. In short, the diameter of the guidance path and the diameter of the bar must closely match each other so that there is a proper fit between the bar and the guide to insure proper functionality of the guide.
When the mill decides to roll a new bar having a diameter smaller than the diameter of the guidance path on the existing guides, the mill must exchange the existing guides for different guides having a guidance path diameter matching the diameter of the new bar.
To reduce the cost and time required to roll different bar sizes, mills use guides that have a guidance path that is large enough to accommodate a range of bar diameters. This permits one guide to handle more than one size bar and therefore minimizes the number of times the mill must exchange guides. However, mills must make a difficult trade-off to both minimize costs and maintain productivity and quality.
If the size range of the guide is too narrow, more guide changes will be required and there will be a greater possibility of undesirable scratches on the bar surface from contact between the bar and the guide. But, if the size range is too wide, a guide will not be function well and undesirable bar motion will occur.
Cobbles:
Furthermore, if the leading end of the bar is not aligned with the guidance path (“bar misalignment”) when the bar enters the guide, the bar will physically collide with the guide. A collision between the bar and the guide significantly increases the amount of friction on the bar, causing the leading end to lose momentum. At the same time that the leading end slows, the rear part of the bar continues to move at the original bar speed. This creates stress on the inside of the bar. Not infrequently, the bar buckles as a consequence. If the bar buckles, the linear motion of the bar stalls. In hot rolled bar mills, this buckling phenomenon is referred to as a “cobble.”
Cobbles can also occur if the leading end of the bar is not properly aligned with the entry to the subsequent device, such as a roll stand or a guide, when the bar approaches the subsequent device. This can result in a collision between the bar and the device. When the bar collides with the device, it can buckle and result in a cobble. Cobbles are wasteful and can be dangerous to both personnel and equipment located near the cobble event because of the heat, motion and mass of the bar.
Surface Quality:
The quality of the surface finish of a bar can be very important to the end-user of the bar product. Many users pay a premium price for bar with high surface quality. Instruments such as eddy current and optical sensors are used in-line at bar mills for quality assurance to detect surface defects on bar as it is being produced. The amount of non-axial motion of the bar affects the detection capability of these sensor devices. Therefore, to enable both eddy current and optical sensors to operate more effectively, guides are used in front of these sensors to minimize the amount of the non-axial movement of the bar.
Bar End Capture:
In order for the guide to function properly, it must first physically capture the leading end of the bar (“leading end”) as it approaches and enters the guide and second it must direct the leading end onto the guidance path. If the opening to the guide is relatively small, the leading end of the bar may not line up properly with the opening and the bar may cobble. To avoid the potential of cobbling, some existing art employs active control systems to control the guides to capture the leading end of the bar. These systems allow the guides to be disengaged from the bar path by actuators, such as pneumatic arms, when the leading end approaches the entry to the guide. Once the leading end is in the guide, the actuators bring the guide into position and engage the guide with the bar. Even with this technique, the guides may still need to be changed frequently to accommodate the tolerances required by different bar sizes.
Prior Art Guide Designs:
Prior art involves a number of different guide designs meant to accomplish some, or all, of the following objectives: (1) to capture the leading end of the bar and (2) to constrain the non-axial motion of the bar. Prior art also frequently attempts to minimize the friction between the bar and the guide and to cool the guide. These guides have a guidance path with a constant diameter.
The simplest guide is a one-piece design illustrated in FIG. 3. The guide is used to constrain the motion of the bar (FIG. 3, item 10), traveling from left to right through the guide. (FIG. 3, item 122.) This diameter must be large enough to accommodate the bar being processed but small enough that the bar moves in the desired manner along the bar path. The guide has an opening that is larger than the guidance path. The inlet angle θ (FIG. 3, item 124) is typically set between 15° and 30° such that the leading end of the bar can be forced onto the desired bar path. One or more such guides can be arranged together to function in tandem. The bar is forced by the guide opening to move onto the desired bar path. This design is efficient at capturing the leading end of the bar and at constraining the non-axial motion of the bar, but does not efficiently minimize the friction between the bar and the guide. Further, these guides are not always easy to align and may not be easy to inspect and maintain due to the limited visual access to their inner diameter surfaces.
A second type of guide has a fixed lower portion and a re-movable upper portion, item 120′ in FIG. 4. The parting line (FIG. 4, item 126) divides the upper and lower portions of the guide. A mechanism, such as a C clamp, is employed to lock the two pieces together to form the guide. The re-moveable upper portion of the guide permits access for maintenance and inspection purposes. In addition, the fixed lower portion typically incorporates a water system to cool the guide. One or more such guides can be arranged together to function in tandem. These guides have an opening that is larger at the front end to efficiently capture the leading end of the bar and force the bar to move onto the bar path. However, this second type of guide does not efficiently minimize the friction between the bar and the guide and it is still necessary to change guides to accommodate different bar sizes.
A third type of guide, illustrated in FIG. 5, uses two or more roll shaped guides, operating in combination. The guides, item 208, have retaining grooves shown as item 210, which have fixed radii. The sum of the radii of the said retaining grooves equals to diameter of the guidance path formed by the retaining grooves. The guides are mounted on supporting arms, item 206. The guides can rotate on their axles, item 212. Mechanical bearings support the said axles allowing them to rotate easily in order to minimize the friction between the bar and the guides. The supporting arms are mounted to the ground structure, item 200, through supporting joints, item 202.
The supporting arms can be manipulated through actuators, item 204 to change the position of the guides relative to the approaching bar (item 10.) This type of guide can be opened up (FIG. 5 (a) item 214) to capture the leading end of the bar, then closed (FIG. 5 (b) item 214′) once the bar is in the guide.
This guide design allows for water-cooling the guides and for easier maintenance.
Tradeoff:
The current art guide designs force the mill operator to make a tradeoff between functionality, i.e. controlling the motion of the bar, and the cost of such functionality, i.e. deciding on the number of guide exchanges that need to be made to achieve such functionality. Guide exchanges take time and require labor. The more guide exchanges required, the higher the mill's operating costs. Closer tolerances between the diameter of the guidance path and the diameter of the bar enhance the guide's functionality. Closer tolerances mean that the guide better serves its main purpose of controlling the motion of the bar. However, if the tolerance is very tight, the mill will have to exchange guides more frequently, and incur more costs, whenever it changes the size of the bar being processed. On the other hand, if the tolerance is set too loose in order to minimize the need for guide exchanges and hence costs, the non-axial motion of the bar will not be as well constrained and the functionality of the guide will be compromised.
In addition, prior art is based on applying force through contact between the guide and the bar to control the non-axial motion of the bar. Such contact, particularly when there is high bar speed and tight bar diameter constraints, has the potential to negatively affect the surface quality of the bar being rolled.